ABSTRACT:
Under high horizontal in-situ stress conditions traditional stress measurement method such as hydraulic fracturing and overcoring often do not give reliable values for the horizontal in-situ stress magnitudes.In this study, a correlation between the magnitude of crack closure energy and the maximum in-situ stress magnitude was established at a site where the stress magnitudes were known with accuracy to depth of 900 m. The crack closure energy was measured on standard uniaxial tests. The correlation was used to estimate the possible range of the maximum horizontal stress at depths between 400 and 700 m at a site in central Sweden. The predicted maximum in-situ stress was in agreement with estimates made using over-core measurements at shallower depths.
1 INTRODUCTION
The Swedish Nuclear Fuel and Waste management Co. (SKB) is currently conducting extensive site investigations at Forsmark, Sweden, one of the candidate sites for construction of a nuclear waste repository. One of the parameters required for the design of the underground openings associated with the nuclear waste repository is the in-situ stress magnitudes and orientations. The repository is currently planned for a depth between 400 and 700 m and hence the in-situ stress magnitudes are required at these depths. Extensive stress measurements were conducted in the 1970’s and 1980’s in the Forsmark area to a maximum depth of 500 m for the construction of the nuclear Power Plants and the final repository for reactor waste, SFR. The SFR Facility includes a series of underground caverns and a 70 meter high underground silo that was constructed in the mid 1980s. This construction experience and the previous stress measurements indicated that the horizontal stress magnitudes at the Forsmark site were greater than the weight of the overburden which is common in the Scandanivain Shield at shallow depths.
